A variety of data communications systems exist for coupling data between end users. Such systems utilize a number of different topologies and protocols. One commonly used arrangement is the Ethernet system in which end users are all connected to one another via a coaxial cable. Deployment of this system has waned due to the need to install coaxial cable. On the other hand, systems which utilize twisted pair wires, such as the 10 BaseT system, have experienced widespread growth. In any of the foregoing systems, information is coupled in bursts between end points. The time interval during each burst is referred to as the busy mode while the time interval between bursts is referred to as the idle mode.
Most recently, a higher-speed version of the 10 BaseT system, known as 100 Mbps Fast Ethernet, is being proposed which increases the nominal data rate tenfold. To lessen implementation costs, this proposed system is compatible with the half-duplex Ethernet and 10 BaseT protocol. However, the higher speed in this proposed system necessitates the use of distortion compensation devices, such as automatic gain control (AGC) circuits, equalizers and NEXT (near-end crosstalk) cancellers.
Equalizers and NEXT cancellers have long been used for distortion compensation and implemented in either of two classes. In the first, the amount of distortion compensation provided is fixed, while in the second, the distortion compensation provided varies to match the time-varying distortion introduced in the communications channel. Due to the varying nature of the distortion compensation provided, this second class of devices is referred to as being "adaptive".
Adaptive NEXT cancellers and equalizers which theoretically provide the "exact" amount of compensation required at any time do so through an adjustment process referred to as convergence or adaptation. The convergence process may be performed either periodically or continuously. In the periodic adaptation process, transmission of customer data is inhibited and "training" signals or sequences which are a priori known to the receiver are transmitted, while in the continuous convergence process the actual customer data is used to update the adaptive devices. Because the customer data values are not a priori known to the receiver, this process is commonly referred to as "blind" convergence or adaptation. With either form of adaptation, there is always some amount of distortion, albeit small, which remains even after convergence.
Due to the widespread use of adaptation devices and the burgeoning growth of system applications requiring ever more stringent performance objectives, a technique which overcomes the shortcomings of the prior art would be desirable.